fdo#78900 RTF import: allow \dpxsize inside \dptxbxtext

Previously we assumed that they always appear under \do directly.

Change-Id: Ie3a978da856986f604a3815d6127abb4191323c0
(cherry picked from commit 21c11ffe254612ac46aa891addacb0024d5b93f0)
Reviewed-on: https://gerrit.libreoffice.org/9488
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>